Heretofore, steel sheets having zinc-based plating such as zinc-plated ones and zinc-aluminum-plated ones have found wide applications in industries which produce domestic appliances, automobiles and building materials. Such steel sheets with a chromate coating applied on a plated face, or with an organic layer further disposed on the chromate layer for improving corrosion resistance have been commonly used. When the organic layer is used, the chromate layer performs another role in that it forms a strong bond thereto.
The above-mentioned chromate layer is highly resistant to corrosion and can bond easily to coating compositions. However, this chromate layer has a drawback in that because it contains hexavalent chromium, it is required that an extra draining treatment be performed at the chromate coating step as provided by the Japanese Water Pollution Prevention Law, and consequently, high costs are entailed.
To prevent white rust from developing on steel sheets, zinc based-plated ones in particular, a demand for the development of a technique of surface-treating such steel without the need for chromium has arisen. To this end, a number of proposals as, for instance, the ones described below have been presented.
1. In Japanese Unexamined Patent Application Publication No. 5-195244, a process for surface-treating a metal in which a chromium-free composition is used is proposed. This composition contains (a) an anionic component composed of at least four fluorine atoms and at least one element of titanium and zirconium (fluorotitanic acid, represented as (TiF62−), for example), (b) a cationic component such as cobalt or magnesium, (c) a free acid for pH adjustment, and (d) an organic resin. However, the surface-treated metal sheet obtained by this process is explicitly stated to exhibit corrosion resistance when such metal sheet is further coated on its upper layer with protective compositions for priming and topcoating as is conventionally practiced. For this reason, the upper layer formed by the surface treatment, when used alone, cannot be said to be sufficiently resistant to corrosion.
2. In Japanese Unexamined Patent Application Publication No. 9-241856, a process for surface-treating a metal in which a chromium-free composition is used is proposed. This composition contains (a) a hydroxyl group-containing copolymer, (b) phosphorus, and (c) phosphates of metals such as copper and cobalt. The surface-treated metal sheet obtained by this process is superior in bare corrosion resistance in as-worked state and an adhesiveness to coatings. On the other hand, there is difficulty in ensuring the conductivity of this metal sheet because it has formed thereon a layer of a dense structure resulting from crosslink between the resin and the different metal phosphates.
3. In Japanese Unexamined Patent Application Publication No. 11-50010, a surface-treating agent for use in a metal is proposed which surface-treating agent is formulated with a chromium-free composition. This composition contains (a) a resin having a polyhydroxy ether segment and a copolymer segment of unsaturated monomers, (b) phosphoric acid, and (c) phosphates of metals such as calcium and cobalt. The surface-treated metal sheet obtained when such an agent is used is superior in bare corrosion resistance, but is not readily conductive because it has formed thereon a dense layer resulting from crosslink of the resin with the different metal phosphates.
4. In Japanese Unexamined Patent Application Publication No. 11-1069450, a water-soluble surface-treating agent is proposed which is prepared by dissolving in an aqueous medium (a) polyvalent metal ions such as of manganese and cobalt, (b) acids such as fluoro acid and phosphoric acid, (c) a silane coupling agent, and (d) a water-soluble polymer with a polymerization unit of 2 to 50. The surface-treated metal material obtained by the use of such an agent is provided thereon with a slightly soluble resin layer disposed to maintain corrosion resistance. To form this resin layer, the metal surface is etched with the aid of the acid components contained in the treating solution. Since the resin layer is composed predominantly of a resin component, conductivity is difficult to attain.
5. In Japanese Unexamined Patent Application Publication No. 11-29724, a process for coating an aqueous rust preventive on zinc-coated steel is proposed. This rust preventive contains (a) a thiocarbonyl group-containing compound, (b) a phosphoric acid ion, and (c) water-dispersible silica. A sulfide such as the thiocarbonyl group-containing compound used in the process of item 5 is, in itself, likely to be easily adsorbed on the surface of a metal such as zinc. In addition, when placed together with a phosphoric acid ion, a thiol group ion of the thiocarbonyl group-containing compound is adsorbed at active sites on the zinc surface during coating of the rust preventive. Thus, rust can be effectively prevented. The zinc-coated steel or non-coated steel obtained by this surface-treating process is highly resistant to corrosion when covered on the surface with a layer structured to have a ═N—C (═S)— group or a —O—C (═S)— group, but on the other hand, is not conductive as a whole. If such a layer is made thinner in order to achieve conductivity, portions of the layer, which have not been covered with the thiocarbonyl group-containing compound, appear eventually causing rust. Thus, corrosion resistance and conductivity performance cannot be well balanced even with the process noted here.
In the processes of items 1 to 4 above, corrosion resistance is obtained to a fairly good extent when a sufficient amount, that is, a sufficient thickness of surface-treating agent (a covering agent or a coating agent) is applied to a metal sheet. However, corrosion resistance is extremely low, for example, when the layer is disposed on a metal sheet with nodules which are partly exposed from the layer, or when the layer of the coating formed is too thin. In other words, corrosion resistance is regarded as acceptable only when there is 100% coverage of the metal sheet by the surface-treating agent, but corrosion resistance insufficient when the coverage is less than 100%. In the case of the above-mentioned surface-treating agents, particularly those of items 2 to 4, a dense resin layer is formed by crosslinking a resin and metal salts in order to obtain corrosion resistance. When disposed as a whole to a greater coating thickness, this resin layer becomes less conductive. To enhance conductivity, the coating thickness may be reduced, but this poses a problem in that the resulting resin layer becomes less resistant to corrosion.
Moreover, all the conventional art cited above in items 1 to 5 are based on the conception that a strong bond should be formed at an interfacial boundary between the surface of a metal and the layer to be derived from a surface-treating agent. From a microscopic perspective, the surface-treating agent cannot be completely bonded to the metal surface, and as a result, there is a limit to how much the bondability can be improved. In enhancing corrosion resistance, therefore, the above conventional art focuses on improvements in the denseness of a resin layer to be derived from a surface-treating agent, but not on the bondability between a surface-treating agent and a metal surface. But improved density and improved conductivity are contradictory requirements.
In office appliances such as personal computers and copiers, as well as household appliances such as air conditioners, there has recently been a demand for a surface-treated steel sheet that is not only devoid of chromium and resistant to corrosion, but which has a low surface electrical resistance. The reason for this demand is that steel sheets with low surface electrical resistance, i.e., steel sheets with good conductivity, are effective in preventing the leakage of noise due to electromagnetic waves. Although many proposals for the surface treatment of metals without reliance on chromium are known, none of them discloses a steel sheet having surface-treated zinc-based plating which can meet requirements of both high conductivity and corrosion resistance.
Taking into account the foregoing situation of the known art, the present invention provides a steel sheet having surface-treated zinc-based plating that needs no extra draining treatment at the step in which a surface-treating agent is coated and at the time the resulting steel sheet is put to practical use and has overcome the defects experienced in the known art. In particular, an object of the invention is to provide a steel sheet having zinc-based plating having formed thereon a surface-treated layer, and which has excellent conductivity and corrosion resistance.